Diagrammatically: odd case (let n be 7) *** *** *** 1 7 6 2 *** *** 1 7 6 2 *** 3 5 1 7 6 2 4 3 5 1 7 even case (let n be 8) 1 *** *** *** *** *** *** 2 1 3 *** *** *** *** 4 2 1 3 5 *** *** 6 4 2 1 3 5 7 8 6 4 2 And their reverses.